Steady state

At equilibrium, the end-tidal concentration approximates the alveolar concentration, which, in turn, approximates the anesthetic concentration at the anesthetic site of action in the central nervous system. Equilibrium is present when end-tidal, alveolar, blood, and brain anesthetic partial pressures are equal. Based on the high cerebral blood flow and low blood solubility of modern anesthetic agents, equilibration is approached after end-tidal concentration has been kept constant for 10 to 15 min. For example, if at sea level and at equilibrium end-tidal N₂O concentration is 60%, then its partial pressure in the alveolus, blood, and brain equals 0.6 atm or 456 mm Hg.

Ambient pressure

MAC values are conventionally given as a percentage of alveolar anesthetic concentration at 1 atm. They either have been determined at sea level or, ideally, have been corrected to sea level when determined at higher altitudes. One has to bear in mind that anesthetic potency and uptake are directly related to the partial pressure of the anesthetic agent (see Table 62-1). At higher altitude, as compared with at sea level, the same concentration of an inhalation anesthetic agent will exert a lower partial pressure within the alveolus and, consequently, a reduced anesthetic effect. Modern variable bypass vaporizers compensate for this effect because, although the dials are marked in “percent,” partial pressure is what is actually determined. At an altitude at which the pressure is one half of sea level, a variable bypass vaporizer set to 1% would deliver 2%, though the actual partial pressure of anesthetic agent delivered would be the same. For example, at sea level, with a barometric pressure of 760 mm Hg, the partial pressure of the agent would be 7.6 mm Hg; at an altitude with a barometric pressure of 380 mm Hg, a variable bypass vaporizer set at 1% would actually deliver 2% of the agent (2% of 380 = 7.6 mm Hg partial vapor pressure).

Stimulus